Thin film transistors (TFTs) are widely used in electronic devices, such as a liquid crystal display (LCD), to serve as a switch component. Generally, a TFT can include a gate, a source, a drain, and a channel layer coupling the source to the drain. The channel layer can be made of amorphous silicon (a-Si), low temperature poly-silicon, LTPS), metal oxides, or other like materials. The TFTs having a lightly doped drain (LDD) structure are widely used because they have the advantage of reducing leakage current.